The invention relates to the field of light amplifiers, and, specifically, to the art of fiber optic evanescent field amplifiers using lasing dyes.
It is well known that certain classes of molecules can be excited to higher level energy states, and that they will emit light when they return to lower energy states. This principle has been applied to make lasers and light amplifiers. Some of these devices use neodymium doped fiber optic waveguides and others use lasing dyes. Light energy from a pump light source is coupled to the material to be excited either by transverse pumping or by end pumping to provide the excitation energy. Transverse pumping means the excitation energy is applied to the active medium from outside the fiber from a direction other than the direction of travel of light in the fiber. End pumping means the pumping signal is coupled into the end of the waveguide carrying the output signal of the device. The evanescent field outside the core in these end pumped devices excites the gain medium.
An example of a dye laser device is described by N. Perisamy in "Evanescent Wave-Coupled Dye Laser Emission In Optical Fibers", Applied Optics, Vol. 21, No. 15, Aug. 1, 1982 at page 2693. Perisamy taught dye laser emission into a multimode fiber waveguide. The waveguide was enclosed in a capillary containing a lasing dye which acted as the cladding for the fiber and had a lower index than the fiber core. A laser pumped the dye. The emitted light from the dye was collected in the waveguide.
N. Perisamy and F. P. Schafer taught laser amplification by transverse pumping of a dye surrounding an optical fiber in an article entitled "Laser Amplification In An Optical Fiber By Evanescent Field Coupling" published in Applied Physics, Issue 24, 1981 at pp. 201-203. There a nitrogen laser was used to transversely pump a dye solution external to a multimode fiber carrying a signal to be amplified. The evanescent field of the signal to be amplified triggered emission of light from the excited dye. Another dye laser using the same dye was used to inject the signal to be amplified so that the wavelength of the signal to be amplified matched the wavelength of emission of the excited dye.
Another multimode fiber optic amplifier was taught by H. Injeyan, O. M. Stafsudd and N. G. Alexopoulos in "Light Amplification By Evanescent Wave Coupling In A Multimode Fiber" published by Applied Optics, Vol. 21, No. 11, June 1, 1982 at p. 1928. There a multimode fiber was surrounded by a dye cavity containing a recirculating lasing dye. The dye was temperature controlled, and the fiber was end pumped. The evanescent field of the higher order modes extended into the dye and supplied the excitation energy. As a result, only the higher order modes were amplified, because the lower order modes did not have substantial penetration of their evanescent fields into the dye. The overall gain for the device was approximately 10% because only the higher order modes were amplified and C.W. pumping was used. The higher order modes carry only part of the total input light.
All these multimode devices have the disadvantage of modal dispersion effects. Modal dispersion effects occur in pulses launched down such a waveguide causing the pulses to spread out and lose their original shape such that eventually they become unreadable. Further, in multimode waveguides only the higher order modes have significant penetration of their evanescent fields into the cladding. This evanescent field penetration is key to causing excitation in the end pumped devices and is key in coupling the emitted light from the active medium into the waveguide. However, in multimode fibers only a fraction of the total input light power propagates in these higher order modes, so these devices amplify only a fraction of the total input light.
A single mode waveguide laser was taught by C. J. Koester in "Laser Action By Enhanced Total Internal Reflection" published in the IEEE Journal of Quantum Electronics, Vol. QE-2, No. 9, September 1966 at p. 580. There a passive (i.e., non-doped) core, monomode fiber was clad with neodymium doped cladding having an index lower than the core. The evanescent field of light travelling in the core excites the Nd atoms in the cladding in this device, and emitted light from the excited atoms bound in the cladding caused amplification manifested as a greater than unity reflection coefficient at the core cladding interface. This type of structure however has several disadvantages. First, the presence of the Nd atoms in the cladding causes the fiber to exhibit more loss because of absorption and scattering. Second, only one type of atom is present as the gain medium with only a limited number of electronic energy states. Thus, the wavelengths of light which can be absorbed and emitted is limited to a narrow range of wavelengths related to the energy gaps between the quantum energy states in the atom. Therefore, this device has a narrow useful bandwidth of frequencies which can be amplified.
An end pumped, single mode thin film waveguide dye laser with evanescent field pumping was taught in "Evanescent Field Pumped Dye Laser" by E. P. Ippen and C. V. Shank published in Applied Physics Letters, Vol. 21, No. 7 on Oct. 1, 1972, at p. 301. There a thin film monomode glass waveguide was deposited on a glass substrate, and doubled Nd: YAG laser pump radiation was end coupled into the waveguide by prism coupling. A dye chamber containing a lasing dye was sealed over the waveguide such that the evanescent field from the pump radiation excited the dye molecules lying near the surface of the film. The excited molecules then emitted light when they dropped back to lower energy states which light was coupled by evanescent coupling into the thin film waveguide.
A disadvantage with this thin film prism coupled type structure is the high coupling losses and alignment difficulties created by the prism coupling. Prism coupling causes losses at both ends of the thin film waveguide in launching light in the waveguide and in extracting the output light from the waveguide. Other means of coupling light into integrated optic thin film waveguide are available, but these other methods are also lossy.
Further, glass thin film waveguides are very lossy in terms of propagation losses because of the impurities which are added in the process of making the glass. These impurities cause stresses in the glass which generate absorption and scattering losses, especially where transition elements such as iron or copper are present as impurities. The traditional methods of making glass cause the presence of such undesirable impurities. Such losses are very undesirable for amplification and lasing applications.
The transverse pumped, end coupled devices described above also have the disadvantage that they generate a great deal of noise, because molecules of the active medium which are far away from the core are excited in addition to those molecules near the core. Only those molecules near the core contribute to the amplification, because the evanescent field of the signal to be amplified only penetrates a short distance into the active medium. Only those molecules within the reach of this evanescent field are stimulated to release light in phase with the signal to be amplified to add to the strength of this signal. The molecules outside the reach of the evanescent field absorb energy and do not release it in synchronization with the signal to be amplified, but they do release light energy spontaneously. Thus, these outer molecules waste pump energy. Further, the spontaneous emissions cause noise. That is, the portion of this spontaneously emitted light which is coupled into the waveguide appears as noise. This noise has foiled other workers in the art, because it tends to mask the amplified signal thereby leading these prior workers to believe that good signal to noise characteristics were not possible.
The nonrecirculating active medium embodiments also have the disadvantage that the active medium molecules or atoms eventually enter what is called the "triplet" state. The triplet state is an excited energy level which has a long lifetime. Electrons dropping from the triplet state to the ground energy state will not emit light, but instead they give up energy in the form of non-radiated energy. Because of the long lifetime in the triplet state, the population of molecules in this unuseable state increases over time where the excitation energy is continuous wave or where the excitation energy is comprised of pulses having a spacing which is shorter than the triplet state lifetime. Unless these molecules in the triplet state are swept away from the evanescent field region, the gain of the device will erode over time as the size of the triplet state population grows. Also, bleaching effects can occur.
Accordingly, a need has arisen for a light amplifier that has high gain, low modal dispersion and low loss, and which is easy to use and effective in monomode, fiber optic systems without excessive difficulty in coupling the amplifier into the circuit. Further, the amplifier should have a reasonably large bandwidth, and it should be capable of operating indefinitely without loss of effectiveness, and it should not require excessive pumping power. Further, it should provide a clean output which is free of excessive noise.